1. Technical Field
The present invention relates generally to a system for measuring and visualizing the full field of deformation behavior of a body in terms of displacement and strain, and more specifically, to methodology, algorithms and a corresponding set of tools for the data acquisition, digital image processing, field variable approximation or interpolation and visualization of digital images of a deforming body.
2. Background Technology
Beginning in the 1980's, digital imaging has been used to measure the deformation state of deformable material specimen. These displacement measurement methods have gained significant attention the last two decades, because of the great impact of digital imaging evolution. Modern digital cameras provide a cost effective and highly reliable tool for recording and processing images of an experiment using a personal computer. Experimental mechanics have greatly benefited from those capabilities and some methods have been developed for the determination of displacement and strain fields.
Both pure grid methods and digital image correlation methods have been proposed for providing full-field measurements of displacement and strain.
In pure grid methods, a uniform grid is applied to the surface of a specimen, and the measurement of deformation relies on the motion of the grid. These methods rely on specialized methods for application of the uniform grid. It can be difficult to apply a uniform grid to irregularly shaped bodies, and any inaccuracies in the application of the grid are a major source of errors in the measurement of deformation.
Pure grid methods are described in Sevenhuijsen, P. J., “Two simple methods for deformation demonstration and measurement”, Strain, Vol. 17, pp. 20-24 (1981); Parks, V. J., “Strain measurements using grids”, Opt. Eng., Vol. 21, pp. 633-639 (1982); Sevenhuijsen, P. J., “Photonics for deformations”, Proc 5th Int. Congr. On Expt. Mechanics, SESA, Montreal, (June 1984); and Sevenhuijsen, P. J., “The Photonical, Pure Grid Method”, Optics and Lasers in Engineering, Vol. 18, pp. 173-194, (1993).
Digital image correlation methods are described in Peters, W.H., Ranson, W.F., “Digital imaging techniques in experimental stress analysis”, Opt. Eng. Vol. 21, pp. 427-432, (1982); Bruck, H.A., McNeil, S.R., Sutton, M.A., and Peters W.H., “Digital image correlation using Newton - Raphson method of partial differential correction”, Expt. Mech. Vol. 28, pp. 261-267 (1989); and Cheng, P., Sutton, M.A., Schreier, H.W., McNeill, S.R., “Full-field speckle pattern image correlation with B-Spline deformation function”, Expt. Mech., Vol. 42, pp. 344-352, (2002).
The performance of methods based on Digital image correlation, which rely on an applied speckle pattern, can be highly sensitive to the application method and on the specimen surface. Schreier, H. W. Sutton, M. A., “Systematic errors in digital image correlation due to undermatched subset shape functions”, Expt. Mech., Vol. 42, pp. 303-310, (2002) discusses the sensitivity of the method to very specific qualitative and quantitative characteristics of the speckle pattern.
Additional grid-based methods are described in Sirkis, J. S., “System response to automated grid methods”, Opt. Eng., Vol. 29, 1485-93, (1990) and Andersen, K., Helsch, R., “Calculation of grating coordinates using correlation filter techniques”, Optik, Vol. 80, pp. 76-79, (1988). U.S. Pat. No. 7,377,181 to Christ, Jr. et al. discloses the use of coded marks.
Bremand, F. and Lagarde, A., “Two methods of large and small strain measurement on a small size area”, Proc. SEM Spring Conf. On Expt. Mechanics, Keystone, Colo., USA, pp. 173-176, (1986) discloses a method of applying a Fourier transform of the grid pattern.
Mesh-free methods are described in Andrianopoulos, N. P., “Full-field displacement measurement of a speckle grid by using a mesh-free deformation function”, Strain, Vol. 42, 265-271, (2006), in Andrianopoulos, N. P. and Iliopoulos, A. P. “Displacements Measurement in Irregularly Bounded Plates Using Mesh Free Methods”, 16th European Conference of Fracture, Alexandroupolis, Greece, Jul. 3-7, 2006.
Random-grid mesh-free techniques are disclosed in Andrianopoulos, N. P. and Iliopoulos, A. P., “Strain measurements by a hybrid experimental-numerical method using a mesh-free field function”, Honorary Volume for Professor P. S. Theocaris, Armenian Academy of Sciences, 31-41, (2005) and in Iliopoulos, A. P., Andrianopoulos, N. P., “An Approach to Analyze Errors Introduced in the Random Grid Strain Measurement Method”, Strain, Vol. 46, pp. 258-266, June 2010 (published online November 2008).
Other methods for measuring deformation and strain use laser beams to illuminate the component. These methods typically require application of a special photosensitive film and can require darkrooms or other special test conditions.